Insulator



March 12, 1935. w. A. SMITH 1,994,291

INSULATOR Filed Feb. 2, 1953 Fig.5

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Patented Mar. 12, 1935 UNITED STATES PATENT ()FFIQE INSULATORApplication February 2, 1933, Serial No. 654,828

1 Claim.

5 for an extended period of time without danger of mechanical orelectrical failure.

Another object of the invention is to provide a device of the classnamed which shall be of improved construction and operation.

Other objects and advantages will appear from the following description.

This invention is in the nature of an improvement of the invention ofArthur 0. Austin shown and claimed in his application Serial No.453,180, filed May 17, 1930.

The invention is exemplified by the combination and arrangement of partsshown in the accompanying drawing and described in the followingspecification, and it is more particularly pointed out in the appendedclaim.

In the drawing:

Fig. 1 is an elevation with parts in section showing one embodiment ofthe present invention.

Fig. 2 is a section of the insulator pin taken on line 2-2 of Fig. 1.

Fig.. 3 is an elevation of the upper end of a modified form of insulatorpin.

Fig. 4 is a section on line 44 of Fig. 3.

Fig. 5 is a view similar to Fig. 3 showing another modification.

The dielectric material, such as porcelain, commonly used for insulatorsfor supporting electrical transmission lines, has a high mechanicalstrength in compression but is relatively weak in tension and shear. Toutilize this material to the best advantage, the present inventionprovides a construction in which the load is transmitted from oneinsulator fitting to the other through the porcelain member largely by acompressive force. The insulator fittings being made of metal have adifferent coefficient of expansion and contraction for temperaturechanges from that of porcelain, and provision is also made to preventexcessive stresses being set up in the porcelain because ofthedifferential expansion or contraction of the parts for temperaturechanges.

In the form of the invention shown in Fig. 1, the numeral designates themetal cap, the numeral 11 the porcelain member, and the numeral 12 themetal pin of a suspension insulator. The porcelain member 11 ispreferably provided with a treated sanded surface for engaging thecement by which it is secured to the metal (01. rye-ms) parts, such asurface being described in Patent 1,284,975, granted November 19, 1918to A. 0. Austin. Cement 13 is interposed between the surface of thedielectric member and the cap, and cement 15 connects the dielectricmember to the pin. The upper end of the pin 12 is shaped to form aninverted cone, the surface of which is covered by a helical spring 16wound about the outer face of the cone. The ends of the springs may beattached in any suitable manner. For this purpose, an opening 17 may bedrilled into the pin at the lower end of the conical surface, into whichone end of the spring may be inserted. The spring is then wound insuccessive convolutions about the conical surface until substantiallythe entire surface is covered; the upper end of the spring being securedto the surface or adjacent turn by solder or other suitable attachment.This not only provides a yielding bearing surface for the pin but alsoprovides roller bearings between the pin surface and the cement 15.

In order to prevent the cement from entering the space within thehelical spring and the depressions in the outer surface between thedifferent convolutions, the spring is covered by Wax or other yieldingmaterial and the excess wax is scraped off, leaving a smooth conicalsurface. When the pin is secured in the recess in the dielectric memberby the cement 15, this will form a smooth inner bearing surface on thecement upon which the spring 16 may roll to permit a wedging actionbetween the bearing surface of the pin and the bearing surface of thecement. The entire surface of the pin embedded in the cement is coatedwith this yielding material or otherwise treated to prevent bondingbetween the cement and the pin. This leaves the portion of the pin belowthe bearing surface unattached to the cement so that it is free to movein the direction of the axis of the pin. .By this arrangement, theentire force of the load is transmitted by the single tapered bearingsurface at the inner end of the pin. The inner surface of the cap isalso coated or otherwise treated to prevent bonding between the cap andthe cement.

It will be noted that the cap 10 is provided With a wedging bearingsurface 17 which is substantially parallel to the conical bearingsurface on the pin and disposed in opposed relation thereto. Thedirection of the force transmitted by the dielectric member andinterposed cement is substantially normal to the two opposed bearingsurfaces, and the porcelain between these surfaces is thus put undercompression. If the two surfaces 16 and 17 were both rigid surfaces,there would be an abrupt termination of the compression in thedielectric member at the edges of the zone transmitting the load and thecompressive force would be greatest at the points where the interposedcement is the thinnest. This would bring excess pressure at the upperend of the pin where the edge of the cone approaches closest to thedielectric member and at the lower edge of the cap. The pressure at thelower edge of the cap is relieved by the resiliency of the cap, whichtends to spring out at its lower edge, thus tapering off the pressure atthe edge of the pressure zone and relieving the pressure where it is thegreatest.

The helical spring on the pin accomplishes a similar result because itpermits yielding at any point of excess pressure, thus passing thepressure along to the next adjacent spring and so on over the bearingsurface, eifecting a distribution of the stress. The reason the pressureis greatest where the cement is the thinnest is because the cement formsa resilient strut, transmitting the pressure, and the total resiliencywill of course be proportional to the length of the strut. The porcelainitself is very rigid and has a high modulus of elasticity compared tocement so that the porcelain itself gives but little; and where there isbut a short elastic strut between the fitting'and the porcelain, thisstrut can yield but a small amount so that the greater proportion of theforce will be transmitted'to the porcelain at this point. Where thecement strut is longer, it will act like a long compression helicalspring and will yield sothat only a small proportion of the force willbe transmitted.

If it were not for the yielding surface provided by the spring 16, theforce of the load transmitted by the conical bearing surface would beconcentrated at the upper edge of the surface where the cement strut isvery short. The spring, however, being much more yielding than thecement will greatly offset the difference in length of the cementstruts, and effect a much more uniform distribution of the stress.

The wedging surface both at 16 and 17 permits adjustment of the partsunder the force of the load to compensate for unequal expansion andcontraction of the metal and dielectric members caused by temperaturechanges. If the pin contracts, it will move downwardly, thuscompensating for the relative change in size between the pin anddielectric. If the pin again expands, the outwardly expansive force willtend to raise the pin in its seat to prevent bursting of the dielectricmember. The movement of the pin under this wedging action is greatlyfacilitated by the roller bearing formed by the helical spring. Theroller bearing surface maintains its eifectiveness for an indefiniteperiod and in this respect has a great advantage over any lubricatingmaterial disposed between the surfaces, which is apt to deteriorate intime or to change its characteristics so as to change the coefiicient offriction between the wedging surfaces. It is quite apparent that if thepin should be drawn into its seat during cold weather and should fail toreturn when the temperature rises, the expansion of the pin wouldproduce a bursting action tending to destroy the dielectric member. Thehelical spring prevents this danger, in addition to preventingconcentration of stress.

The bearing surface of the pin may be shaped to compensate somewhat forthe difference in the length of the resilient cement strut by changingthe angle of the surface so as to vary the compressive force transmittedto the cement for different portions of the conical surface.

In Fig. 3 the bearing surface is made convex in the form of an arc. Itwill be apparent that when a load is placed on the pin, tending to drawit into its seat, there will be a much greater movement normal to thebearing surface in the direction of the arrow at the point 18 than therewill be in the direction of the arrow at the point 19. There will,therefore, be a much less compression on the cement strut at the upperedge of the bearing surface than at the lower edge, but since the strutat the upper edge, being shorter, yields a much less amount than thelonger strut at the lower edge, the lesser force at the upper edge willbe more completely transmitted to the dielectric member than will thegreater force at the lower edge of the bearing surface. In this way, thedistribution of stress transmitted to the dielectric member will beequalized over the surface of the dielectric member.

Instead of an are shaped bearing surface, the bearing surface may bedivided up into conical sections having different angles, as shown inFig. 5, in which the bearing surface is broken up into two steps 20 and21; the upper step 20 having a steeper angle than the lower step 21.While the form of bearing surface shown in Figs. 3 and 5 tends toequalize the stress on the dielectric member, it has the disadvantagethat it distorts the direction of the stress so that a portion of thestress is directed toward the part of the dielectric member below theedge of the cap 10, at which point there is no abutment for opposingthis stress. This tends to produce a shear in the porcelain member forwhich the dielectric material is ill adapted.

It should be noted that the entire bearing surface of the pin is locatedat the upper end of the pin on a single wedge shaped step. This aids inpreventing stress in the dielectric member from being directed to aportion of the dielectric member below the lower edge of the cap; andwhere a single conical surface is used, the entire stress may bepractically confined to the portion of the dielectric member backed upby the bearing surface 17 of the cap. Where a pin is fixed to the cementthroughout the portion thereof within the opening in the dielectricmember so that stress is transmitted to the cement throughout thisportion of the pin, either thepin, the stress transmitted by the lowerportion of the pin will be imparted to the portion of the dielectricmember not supported by the bearing surface of the cap, which createsundue shearing stress in the dielectric material. This tendency ismagnified by pin stretch, which tends to concentrate the forcetransmitted at the lower end of the pin. A single wedging surface at theupper end of the pin avoids this tendency and produces an insulatorhaving much greater mechanical strength than where the force isdistributed over the entire portion of the pin disposed in the recess inthe dielectric member. The advantages thus obtained, however, arelargely neutralized if the rigid surby distributed bearing surfaces orby bonding to face of the" pin is permitted to bear upon the cementbecause of the concentration of stress at the upper portion of thetapered bearing surface where the cement strut is short. By combiningthe single wedging surface and the resilient roller bearing, theadvantage of the single step pin is secured and the disadvantage isoifset so that insulators of mechanical strength heretofore impossiblehave been obtained, and by the use of the yielding roller bearingsurface, the danger of failure of such insulators after a lapse of timeis avoided.

In some instances, it is desirable to hold the pin from rotation in thecement. This may be done in a number of ways. In Figs. 1 and 2 the pinis shown as provided with flat faces at 22 and 23 to prevent rotation inthe cement, while in Fig. 3 the pin is provided with an ellipticalportion 24 which serves a similar purpose.

I claim:

An insulator comprising a dielectric member having a boss thereonprovided with a recess, a cap surrounding said boss and having a conicalbearing surface adjacent the rim of said cap, cement interposed. betweensaid boss and cap and bonded to said boss but unbonded to said cap, apin disposed in said recess and having a single conical bearing surfaceat its inner end, a helical spring roller wound in a plurality ofconvolutions on said conical bearing surface, said pin having a recesstherein for receiving the end of said spring roller to hold the end ofsaid roller in position, cement interposed between said pin anddielectric member and bonded to the surface of said dielectric memberbut unbonded to said pin, said cement having a smooth conical bearingsurface thereon conforming to the bearing surface on said pin but spacedtherefrom to accommodate said spring roller between said pin and cement,a portion of said pin in said cement being non-circular to preventrotation of said pin, said pin and cap being sufliciently overlappedthat substantially the entire force of the load on said pin is directedtoward the bearing surface of said cap to impart maximum strength tosaid insulator, said spring roller acting to distribute the load on thebearing surface of said pin and to prevent concentration thereof' at theextremity of the pin where the thickness of the cement between said pinand dielectric member is least.

WILLIAM A. SMITH.

